This invention rlates to a cyclical pressure swing adsorption process for the purification or fractionation of a gaseous stream using six alternating adsorbers. For the sake of simplicity, when "purification" and the like are set forth hereinafter, these terms are intended to include fractionation and the like as well. Accordingly, this invention is applicable to processes wherein the desired products may be either adsorbed or unadsorbed, but to avoid confusion, the product gas herein refers to the unadsorbed gas.
In particular, the invention is directed to a six adsorber system wherein the gaseous stream during an adsorption phaase at the highest process pressure is conducted through an adsorber and purified gas is withdrawn from the outlet end of the adsorber, the adsorber being subjected after termination of the adsorption phase, to a multistage cocurrent expansion with the resultant expansion gases being utilized, in part, for pressure buildup of other, previously scavenged adsorbers and, in part, for scavenging another adsorber which is at the lowest process pressure. The cocurrently expanded adsorber is then subjected to a countercurrent expansion at the lowest process pressure and the adsorber is scavenged with cocurrent expansion gas from another adsorber. The adsorber is then subjected to multistage pressurizing to the adsorption pressure with cocurrent expansion gas and purified product gas.
A process of this type is described in DOS No. 2,851,847. The process scheme illustrated in FIG. 5 thereof relates to an installation with six adsorbers, the adsorption phases of respectively three adsorbers overlapping with one another with respect to time. Cocurrent expansion occurs in two stages, wherein in a first phase, pressure equalization takes place with another adsorber passing through a pressure buildup phase, and in a second phase, scavenging gas is obtained for use in another adsorber, the latter is being scavenged at that time. After the subsequent countercurrent expansion and scavenging steps, a two-stage pressure buildup occurs, first with expansion gas from an adsorber which is in a cocurrent expansion phase at that time and finally with product gas.
Furthermore, DOS No. 2,624,346 discloses a pressure swing adsorption process with nine adsorbers wherein it is likewise important to operate at least three adsorbers simultaneously in adsorption. In this method, within one cycle, four cocurrent expansion stages are provided for each adsorber, three of these stages being carried out in pressure equalization with adsorbers to be pressurized. Finally, there are a number of pressure swing adsorption processes operated with the use of two to a maximum of five adsorbers, with the common feature that only one adsorber at a time passes through an adsorption phase. This is necessary in processes of this type since otherwise the remaining basic process steps canot be performed. For examples of such processes, attention is invited to U.S. Pat. Nos. 3,430,418 and 3,564,816.
Therefore, the known state of the art generally offers the teaching that, with the use of more than five adsorbers, several adsorbers simultaneously are operated in an adsorption phase. This is also expressly set forth in German Pat. No. 3,006,836. The parallel adsorption in several adsorbers was considered to be especially desirable with a view toward the constant quantity of the product gas as well as of the residual gas stream, i.e., the countercurrent expansion gas, as well as the scavenging gas loaded with desorbed components.